Screw rotor

ABSTRACT

A screw rotor for screw vacuum pumps, comprising a rotor shaft which bears at least two displacement elements. The displacement element is conical in the conveying direction and the adjoining conveying element has a cylindrical design.

BACKGROUND 1. Field of the Disclosure

The disclosure relates to a screw rotor for a screw vacuum pump.

2. Discussion of the Background Art

Screw vacuum pumps comprise a suction chamber in a housing, in whichsuction chamber two screw rotors are arranged. Each screw rotorcomprises at least one displacement element with a helical recess.Thereby, a plurality of windings is formed. With screw vacuum pump, thegoal is always to achieve an internal volume ratio that is as high aspossible. The internal volume ratio is the ratio of the volume at theinlet of the vacuum pump to the volume at the outlet of the vacuum pump.Screw vacuum pumps of the first generation, such as e.g. the pumpsLEYBOLD Screwline or BUSCH Cobra, have an internal volume ratio ofapprox. 3 to 4. With vacuum pumps currently on the market, such as e.g.the screw vacuum pumps LEYBOLD DRYVAC or Edwards GKS, the volume ratiois 5 to 7.

To achieve low pressures at the pump inlet, a high energy input isrequired and the power consumption of a corresponding vacuum pump isvery high, respectively.

It is an object of the disclosure to provide a screw rotor for screwvacuum pumps with which it is possible to reduce energy consumption.

The disclosure is based on the finding that it is possible to reduceenergy consumption by increasing the internal volume ratio.

For a high internal volume ratio the outlet stages of the pump must havea small delivery volume. However, small outlet stages have adisadvantageous ratio between transport flow and return flow, i.e. theyare relatively leaky. Thus, only a rather low pressure build-up can begenerated with each single stage. In order to still realize the majorpart of the compression in the small outlet stages, a great number ofoutlet windings becomes necessary.

Basically, two designs of screw rotors are known. These are screw rotorswith either a cylindrical or a conical outer dimension.

With cylindrical rotors, the tooth space width at the outlet must bechosen to be small for high internal volume ratios. Thereby, the toothheight becomes rather large relative to the tooth space width, which canbe realized in manufacture only with great effort and at high costs.However, with this rotor design, it is readily possible to integrate agreat number of small outlet stages in the rotor (if the ratio of toothheight/tooth space width allows for an economically feasiblemanufacture). Thus, the geometrically installed volume ratio alsobecomes effective thermodynamically.

With conical rotors, in contrast, the chamber volume decreases steadilytowards the outlet due to the tapering tooth height, so that it ispossible to produce small outlet volumes with a ratio between the toothheight and the tooth space width that is favorable under manufacturingaspects. However, it is difficult to manufacture a plurality of stageswith the low delivery volume, since the tooth height decreasescontinuously due to the cone shape. Although high geometric volumeratios are thus conceivable, these hardly show the desired effect, sincethe compression occurs in the larger stages due to the return flowthrough the gap.

It is further known to design staged rotors with two cylindricaldisplacement elements having different diameters. However, a greatdisadvantage of these staged rotors is the discontinuous transitionwhich is extremely difficult to manufacture. Therefore, staged rotorsare not common on the market.

SUMMARY

The screw rotor of the disclosure comprises a rotor shaft connected withat least two displacement elements, each displacement element comprisingat least one helical recess. According to the disclosure a suction-sidedisplacement element, i.e. a displacement element arranged in thedirection of the pump inlet, in particular at the pump inlet, isdesigned to taper in the conveying direction. The displacement elementis arranged such that is tapers in the conveying direction, i.e. towardsthe pump outlet. It is particularly preferred that the suction-sidedisplacement element has an outer contour that is designed to constantlydecrease in the conveying direction. A conical design of thesuction-side displacement element, tapering in the conveying direction,is particularly preferred. The cone angle is preferably in a range from2° to 8°.

Further, a pressure-side displacement element, i.e. a displacementelement provided in the direction of the pump outlet, in particular atthe pump outlet, is designed substantially cylindrically. Thepressure-side displacement element may also be designed to be slightlyconical or slightly decreasing constantly in the conveying direction.The substantially cylindrically designed pressure-side displacementelement has, in particular, a diameter ratio from 1.1 to 1.0 between thesuction-side diameter directed towards the pump inlet and thepressure-side diameter directed towards the pump outlet.

If so desired, further cylindrical and/or conical displacement elementscan be arranged on the rotor shaft. The combination of a suction-sidetapering, in particular conical displacement element and a pressure-sidesubstantially cylindrical displacement element is essential according tothe disclosure. This allows to combine the advantages of both screwrotor designs. The tooth height is reduced by the preferably conicallydesigned suction-side displacement element, so that it is possible toprovide the substantially cylindrical displacement element, whichadjoins the former element in the flow or conveying direction, with alarge number of outlet windings with small delivery volumes at a smallratio between the tooth height and the tooth width. In a particularlypreferred embodiment it is thus possible to realize internal volumeratios greater than 6, in particular greater than 8 and particularlypreferred greater than 10.

Although more than two displacement elements may be provided, a conicaldisplacement element arranged on the suction side and a cylindricaldisplacement element arranged on the pressure side are provided in aparticularly preferred embodiment. Hereinafter, the disclosure will bedescribed with reference to this preferred embodiment, while furtherdisplacement elements may be provided in each case.

It is preferred that the adjacent displacement elements in particularabut against each other or contact each other by their end facesdirected towards each other, and that they are substantially equal indiameter. Thus, a substantially stepless transition is realized. Whenproviding two displacement elements, the diameter of the conicaldisplacement element, starting from the pump inlet, decreases to adiameter that corresponds to the diameter of the cylindricaldisplacement element.

It is further preferred that the diameter of the cylindricaldisplacement element is smaller by 5-35%, preferably 10-25%, than thesuction-side diameter of the conical displacement element, i.e. inparticular the diameter of the conical displacement element provided atthe inlet of the pump.

In a particularly preferred embodiment the lengths and the diameters ofthe conical and the cylindrical displacement element are chosen suchthat the greater part of the compression is performed at a low suctionpressure by the cylindrical displacement element. In particular morethan 70% of the compression performance is provided by the cylindricaldisplacement element.

As such it is further preferred that the conical displacement elementhas an internal volume ratio of more than 4, in particular more than 8.

In a preferred embodiment the cylindrical displacement element has aninternal volume ratio greater than 1, in particular greater than 3. Theinternal compression is preferably effected by a decrease in pitch.

In the transition region between the in particular conical displacementelement and the substantially cylindrical displacement element nocontinuous transition of the winding pitch has to be provided. A step inthe winding pitch is also possible so that an internal compression iscaused thereby. The internal compression can thus be caused already inthe transition and/or in the cylindrical part.

For further improvement it is preferred according to the disclosure thatthe ratio between the tooth height and the tooth width is less than 3 inthe outlet region of the vacuum pump. A favorable manufacture ispossible. In particular the ratio is in a range from 1.8-2.2. In anotherpreferred embodiment of the disclosure the length of the substantiallycylindrical displacement element is 25-50% of the total profile lengthof the screw rotor.

In another preferred embodiment the ratio between the outer diameter ofthe displacement element at the pump outlet and the outer diameter ofthe displacement element at the pump inlet is less than 0.9, inparticular less than 0.85.

Moreover it is particularly preferred that the diameter of the taperingdisplacement element is 80-300 mm in the region of the pump inlet. Inthe transition region between the tapering displacement element and thesubstantially cylindrical displacement element, the diameter ispreferably 65-180 mm. Correspondingly, in a preferred embodiment, theouter diameter of the substantially cylindrical displacement element isalso 65-180 mm, wherein in case of a cylindrical displacement elementalso tapering slightly this diameter may also be somewhat smaller thanthe diameter in the transition region.

In a preferred development of the disclosure, the number of windings ofthe cylindrical displacement element is at least 6 or preferably atleast 10 and particularly preferred at least 12. By providing a greatnumber of windings of the cylindrical displacement element the majorpart of the compression can be performed by the same.

In a preferred embodiment the in particular conical suction-sidedisplacement element has 3-6 windings.

The individual displacement elements are preferably formed as separatecomponents and are connected to the rotor shaft e.g. by being pressedthereon. However, it is also possible that individual or alldisplacement elements are formed integrally with the rotor shaft.

The rotor shaft further preferably comprises cylindrical subs at bothends that serve as bearing seats. However, it is also possible tosupport the screw rotor in an overhung manner, i.e. on one side.

Basically the screw rotor of the disclosure can be manufactured fromknown materials, such as steel, cast iron or aluminum, with theadvantage of the disclosure being realizable in particular with screwrotors of steel or cast iron.

In a preferred development of the disclosure a further displacementelement is provided on the suction side. The further displacementelement is thus arranged upstream of the in particular conicaldisplacement element that tapers in the conveying direction. The furtherdisplacement element preferably is an also substantially cylindricaldisplacement element. Here, it is preferred that the pitch of thewindings of this further displacement element decreases in the conveyingdirection.

Further, the disclosure relates to a screw vacuum pump with two screwrotors arranged in a suction chamber defined by a housing. Here, the twoscrew rotors are designed or developed according to the disclosure, asdescribed above.

The disclosure will be explained hereinafter in more detail withreference to a preferred embodiment and to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a schematic side view of an embodiment of a screw rotoraccording to the disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The screw rotor illustrated comprises a rotor shaft 10 supporting twodisplacement elements 12, 14. The two cylindrical ends 16, 18 of therotor shaft serve to receive bearings for supporting the screw rotor ina pump housing. It is also possible to support the rotor shaft in anoverhung manner, i.e. on one side.

The displacement element 12 on the right in the FIGURE is conical andtapers in the conveying direction 22 from a pump inlet 20 which isarranged on the right in the FIGURE but not illustrated therein, towardsa pump outlet 24 which is arranged on the left in the FIGURE but notillustrated therein. A helical recess 26 of the conical displacementelement 12 is designed such that the volume decreases. This is achievedon the one hand due to the conical outer shape of the displacementelement 12 and on the other hand due to the inner portion 28 of thedisplacement element 12 widening in the conveying direction. Individualchamber volumes formed by the two meshing screw rotors thus reduce theirrespective volume in the conveying direction 22.

In the embodiment illustrated in which only two displacement elements12, 14 are provided, an end face 30 of the displacement element 12 whichis directed towards the pump outlet 24 or towards the pressure side ofthe pump, abuts on an end face 32 of the cylindrical displacementelement 14. The end face 32 is directed towards the pump inlet or in thedirection of the suction side of the vacuum pump. The diameters of thetwo displacement elements 12, 14 are substantially the same in theregion of the end faces 30, 32.

The cylindrical displacement element 14 has a helical recess 34 as well.

In the embodiment illustrated the same has a constant pitch, wherein adecrease in pitch is also possible in the conveying direction 22 forfurther compression. Due to the recess 34 8 windings are formed in theembodiment illustrated.

What is claimed is:
 1. Screw rotor for screw vacuum pumps, comprising arotor shaft, at least two displacement elements connected with the rotorshaft, each having at least one helical recess, wherein a suction-sidedisplacement element is designed tapering in the conveying direction,wherein a pressure-side displacement element is designed substantiallycylindrically.
 2. Screw rotor of claim 1, wherein the suction-sidedisplacement element is designed to constantly decrease in the conveyingdirection.
 3. Screw rotor of claim 1, wherein the interior volume ratiois greater than
 8. 4. Screw rotor of claim 1, wherein the displacementelement have substantially the same diameter at the end faces directedtowards each other.
 5. Screw rotor of claim 1, wherein the diameter ofthe substantially cylindrical displacement element is smaller by 5-35%than the suction-side diameter of the conical displacement element. 6.Screw rotor of claim 1, wherein the tapering displacement element has avolume ratio greater than
 4. 7. Screw rotor of claim 1, wherein the atleast cylindrical displacement element has a volume ratio of 1 to
 3. 8.Screw rotor of claim 1, wherein the diameter of the taperingdisplacement element is 80 to 300 mm in the region of the pump inlet. 9.Screw rotor of claim 1, wherein the diameter of the taperingdisplacement element is 65 to 180 mm in the transition region to thecylindrical displacement element.
 10. Screw rotor of claim 1, whereinthe diameter of the substantially cylindrical displacement element is 65to 180 mm in the region of the outlet.
 11. Screw rotor of claim 1,wherein the number of windings of the cylindrical displacement elementis greater than
 6. 12. Screw rotor of claim 1, wherein the number ofwindings of the tapering displacement element is 3 to
 6. 13. Screw rotorof claim 1, wherein a further displacement element is provided that isarranged upstream of the tapering displacement element in the flowdirection, the further displacement element being preferablysubstantially cylindrical in shape.
 14. Screw vacuum pump with a housingdefining a suction chamber and two screw rotors of claim 1 arranged inthe suction chamber.
 15. Screw rotor of claim 5, wherein the diameter ofthe substantially cylindrical displacement element is smaller by 10-25%than the suction-side diameter of the conical displacement element. 16.Screw rotor of claim 6, wherein the tapering displacement element has avolume ratio greater than
 8. 17. Screw rotor of claim 11, wherein thenumber of windings of the cylindrical displacement element is greaterthan
 10. 18. Screw rotor of claim 17, wherein the number of windings ofthe cylindrical displacement element is greater than
 2. 19. Screw rotorof claim 3, wherein the interior volume ratio is greater than
 10. 20.Screw rotor of claim 19, wherein the interior volume ratio is greaterthan 12.